What is Responsivity?
Responsivity is a measure of input-output gain of the detector in a fibre optic system. In the case of a photodetector, it is the measure of electrical output per optical input. Mostly photodetectors are linear functions of input incident power. The responsivity of a photodetector is typically expressed as units of amperes per watt or volts per watt of radiant power.
It is typically highest in a wavelength region where the photon energy is above bandgap energy, and then declining sharply in the region of bandgap, where the absorption decreases. It can be derived as follows:-
Rλ = Responsivity
IP = Output Photo Current
P = Incident Optical Power
Since hυ = Energy of Photon,
P = rp hυ
Where rp = Photon flux = P/hυ = Photons/sec
Hence the electron rate comes out to be,
re = ηrp = ηP/hυ
Therefore, the output current is
Ip= eηP/hυ = eηλ/hc = eηλ/1.24
h = Plank’s Constant = 6.63 × 10-34 joule-sec
Responsivity of Typical Photodetectors
The relationship between responsivity and wavelength can be understood from the given table below. The unit of wavelength is in nanometers and the unit of responsivity is ampere per watts.
The responsivity of the photodetector depends on the wavelength of incident radiation and the bandgap of the material. The below-given diagram shows the responsivity vs wavelength graph. The different range of responsivity is shown: maximum, average, and minimum.
Question 1:The quantum efficiency of the photodiode is 50% with a wavelength of 0.90* 10-6. Determine the responsivity of photodiodes.
Answer: 0.36 A/W
Question 2:Determine the wavelength of the photodiode having a quantum efficiency of 40% and Responsivity of 0.304 A/W.
Answer: 0.44 μm μ
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